Sequential development of the liver lesions in new-born lambs infected with Rift Valley fever virus. I. Macroscopic and microscopic pathology.

Ten new-born lambs were necropsied at various intervals after artificial infection with Rift Valley fever virus for the study of the sequential development of the hepatic lesions. During the late stage of the disease, the livers were slightly to moderately swollen and mottled yellow, orange-brown and red. Greyish-white foci, approximately 0,25-0,5 mm in diameter, were also scattered throughout the parenchyma. Microscopically, the liver lesions progressed from sparsely distributed acidophilic bodies and hepatocytes, revealing acidophilic degeneration and necrosis 6-12 h post-inoculation, to small randomly scattered primary foci of necrosis 12-24 h after infection. At 30-36 h, these primary foci were larger and more circumscribed. In addition, numerous acidophilic bodies and necrotic hepatocytes were dispersed throughout the markedly degenerated parenchyma. The terminal stage of the disease (48-53 h after inoculation), was characterized by massive hepatic necrosis in which primary foci of necrosis could still be recognized as dense aggregates of cytoplasmic and nuclear debris.